Try the following. Press and hold the power button for 3 seconds then, release it. The NUC will display the option to get into the BIOS. I suggest you to reset it to defaults. It is easier with a speaker or headphones; after the third beep release the button.
If problem continues, I suggest you to recover the BIOS with a update. It requires to open the chassis, remove the BIOS jumper, download the .BIO file.
Latest BIOS files at
Intel® NUC BIOS Recovery Update Instructions